1. Please provide complete code, correctly marked up and spoilered.
2. Please don't rely on testing in some webbrowser when the JavaScript version in use on CW is Node, and not any webbrowser's. Test in Node ( and preferably the correct version for the kata ).
3. "This .. checks .. not alphabetic"; "It check[s] .. non letter"; you are agreeing with him - what is the problem?

It's better to provide a complete code and correct code, otherwise one needs to guess what's missing.

This only checks if one of a or b is not alphabetic, which is the condition for returning -1.

I totally agree. Once I started writing the steps down, I realized it was far less complicated then I initially thought.